The variable pulley transmission, or continuously variable transmission (CVT) as it is commonly called, has been under development for over two decades. Its use has evolved from industrial and marine applications to automotive applications, not in aircraft electrical systems. Its control is rather complicated and is very sensitive to operating conditions. For example, in a control arrangement for a variable pulley transmission disclosed in U.S. Pat. No. 4,458,318, a variable line pressure is controlled and applied to the sheave of the secondary or driven pulley of the transmission, and also serves as the hydraulic power supply to a ratio control valve in a speed control loop of the transmission.
It has been found that due to the variable supply pressure in the continuously variable transmission of U.S. Pat. No. 4,458,318, both the pressure control valve (a pressure relief type valve) and the ratio control valve gains change with the supply pressure, and the control is therefore very sensitive to system operating conditions. There is a need for an improved continuously variable transmission with a control arrangement and method which overcome these drawbacks and disadvantages of this known continuously variable transmission. More particularly, there is a need for an improved continuously variable transmission with a control arrangement and method which are simpler and more robust than those disclosed in U.S. Pat. No. 4,458,318.
It is an object of the present invention to provide a continuously variable transmission with a control arrangement and method which meet this need. The commonly owned U.S. patent application Ser. No. 09/222,958 for CONTINUOUSLY VARIABLE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR REDUCING TRANSMISSION BELT SLIPPAGE, and U.S. patent application Ser. No. 09/222,948 for CONTINUOUSLY VARIABLE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR PREVENTING TRANSMISSION BELT SLIPPAGE, both filed Dec. 30, 1998, and hereby incorporated by reference, also address this need.
One of the main problems in the application for a continuously variable transmission as a constant speed drive for electrical power generation is the prevention of the belt or chain from slipping following the application and removal of a large shock load. There are two approaches for solving this problem: one is to increase the belt or chain clamping force and the other to limit the load torque transmitted to the pulley using a clutch, as disclosed in the aforementioned copending U.S. patent application Ser. No. 09/222,948, for CONTINUOUSLY VARIABLE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR PREVENTING TRANSMISSION BELT SLIPPAGE. Both of these methods require complicated hardware design. The present invention discloses and relates to a continuously variable transmission and method for reducing impact of shock load, e.g., for "softening" the load impact on the CVT pulley to give sufficient time for the control to react to the load to prevent belt slippage. The invention covers transmissions with controls using both the "push-belt" like belts, as well as for "pull-belt" style chains. Thus, the term "belt" as used herein is intended to encompass both these types of drive transmitting devices for continuously variable transmissions.
A continuously variable transmission according to the invention for transferring drive from an engine to a device to be driven, particularly an electric generator to be driven at constant speed, comprises a primary pulley for receiving drive from an engine, a belt, and a secondary pulley which is coupled via the belt to the primary pulley for transferring drive to the device to be driven. The primary and secondary pulleys each have an axially movable sheave and a hydraulically operated actuator therefor to effect ratio change of the transmission and to maintain belt tension. A single source of constant hydraulic pressure is operatively connected for driving each of the actuators. A torsional spring damper device or elastomer damper device is provided between the device to be driven and an output shaft of the secondary pulley of the continuously variable transmission for introducing a time lag in the transmission of torque between the device to be driven and the output shaft of the secondary pulley of the transmission in response to a sudden load change. This transmission is particularly advantageous in that the torsional spring damper device or elastomer damper device can be used to replace the relatively more complicated and bulky controllable clutch. The time lag gives sufficient time for the control system to respond to the sudden load change.
In the ideal case, the rate of torque transmission should match the response of the control system. To compensate for the difference between the two due to design and/or operating temperature change, according to a second feature of the invention, the hydraulic pressure control loop of the transmission can include a lag element for compensating for a difference between a rate of torque transmission between the device to be driven and the transmission, and a response rate of the hydraulic pressure control loop to sudden changes in load of the device to be driven on the transmission. The lag element preferably has a time constant which changes with temperature of the hydraulic fluid of the hydraulic pressure control loop to compensate for a variation of hydraulic fluid viscosity. This second feature of the invention can be used in combination with the aforementioned feature, or independently thereof.
The continuously variable transmission of the present invention comprises a further feature wherein a ratio control valve operatively connects a source of hydraulic pressure to one of either of the actuator of the primary pulley or the actuator of the secondary pulley, depending on operating conditions, to effect the ratio change of the transmission while the ratio control valve operatively connects the other of the actuators of the primary and secondary pulleys to a return. A pressure control valve of the transmission operatively connects the source of hydraulic pressure to the ratio control valve for controlling the return pressure of the ratio control valve to maintain the belt tension. This control arrangement of the transmission, therefore, includes two interactive control loops: a pressure control loop to provide enough belt clamping force for the primary and secondary sheaves to prevent the belt from slipping, and a speed control loop to change the CVT transmission ratio in order to maintain a constant rotational speed of the generator during engine speed and load variations.
A method for controlling a continuously variable transmission according to the invention comprises setting the hydraulic pressure to be applied by the hydraulic pressure control loop to keep the belt from slipping. This step of setting the hydraulic pressure to be applied includes determining the load torque transmitted from the device to be driven by the secondary pulley of the transmission and the speed ratio of the transmission, calculating a value P.sub.set for the set pressure of the hydraulic pressure control loop utilizing the determined load torque, and multiplying the calculated value for the set pressure P.sub.set by a margin factor slightly greater than 1, such as 1.2, to prevent belt slippage, while guarding against the use of too large a pressure which would shorten the life of the belt and the pulleys. During the transient period of a sudden increase of the load torque on the transmission, a greater factor is utilized for determining the set pressure than during the period the load torque is at a steady state value, whereby belt slippage does not occur.
These and other objects, features and advantages of the present invention become more apparent when taken in conjunction with the following detailed description of a preferred embodiment in accordance with the invention and the accompanying drawings.